I contacted Seachem directly a while back concerning the effect of Prime on nitrite, and their response clearly stated that Prime is only effective for 24-48 hours. After this period, the nitrite will again become toxic if it still remains (i.e., has not been handled by bacteria).
Here is the response verbatim from their email:
The nitrite is still completely available for the beneficial bacteria in the bio-filter to consume it. After 24-48 hours, when the Prime is no longer active, if the nitrites are still present and have not been consumed by bacteria, then yes, they will be re-released. Unfortunately, there is not a test kit on the market that will distinguish between the bound form of nitrites and nitrates and the unbound form. So the best option is to continue to add Prime every 48 hours until you are able to get a zero reading for nitrites. Most ammonia test kits out there are the same in that they cannot distinguish between free ammonia and total ammonia, simply giving you one reading for total. This can be misleading, especially when you are trying to cycle a new tank. Both our Multitest: Free and Total Ammonia and our Ammonia Alert, can distinguish between the two, making it much easier to know exactly what is going on inside your aquarium.